State of the art airborne navigation systems continue to emphasize the use of microwave Doppler radar techniques; basically, a microwave signal is transmitted from a moving platform to a reference surface in order to detect such movement by then sensing the frequency shift such movement produces in the radiation transmitted to and reflected from the surface. This shift (.DELTA.F) is due to the Doppler effect and is proportional to the relative velocity between the transmitter and the surface, modified by the transmission angle. It is defined by the Doppler equation: EQU .DELTA.F=2V/.lambda. cos .phi..sub.s ( 1)
where V is the velocity vector relative to the surface and .phi..sub.s represents the angle between the velocity vector and the instantaneous radar transmission angle, independent of the radiation's angle of incidence at the surface.
These systems can be used to derive the three dimensional velocity vector from the sensed movement: heading (V.sub.H); drift (V.sub.D) and altitude (V.sub.E). The heading and drift vectors are coplanar and perpendicular, in quadrature with each other; the altitude vector is perpendicular to the plane.
These systems typically use three or more separate microwave antennas and receivers to resolve the velocity vector. The system output is spatially multiplexed in angle by the fixed antenna spray so as to resolve the velocity vector. For example, two microwave antennas used for sensing the heading and drift movement would be arranged in quadrature pointing towards their respective directions. Additional parameters can be resolved from the obtained velocity: such as, acceleration (by differentiation) and displacement (by integration).
These systems have several significant disadvantages. Using microwave frequencies the best possible resolution is typically no better than 20-30 cm/sec; this is due, in part, to the large defraction angle at microwave frequencies. At these wavelengths the return characteristics (noise, for example) of the signal are highly dependent on the surface characteristics, which can cause dramatic changes in the return signal. In addition, there is significant radial defraction spread at these frequencies which facilitate the detection of system operation; this lack of covertness may limit the utility in hostile areas.
An optical system, one using a laser, has distinct advantages in the above respects and is therefore an attractive alternative. Its much shorter wavelength can provide superior resolution and the signal return is less dependent on surface characteristics. The laser beam may be very narrow and has minimal defraction spread; it is therefore not easily detected. However, it is expensive and complicated to use multiple lasers, antennas and detectors, and so, even though a more precise navigation system, one overcoming the pitfalls of a microwave radar system, may find solution in the laser technology, that technology has not clearly presented an attractive way for sensing three dimensional movement in an economical, practical way. The fact is that for laser technology to be useful in these systems there is a need for an arrangement by which a laser Doppler shift can be ascertained, in the required three dimensions, by using only one laser beam, for such a system can be small, light and cost attractive.
In the paper I coauthored with R. L. Delboca, The Guidance and Control of Helicopters and V/STOL Aircraft at Night and in Poor Visibility, (October 1978), reprinted from the Conference Proceedings No. 258 of the Advisory Group for Aerospace Research and Development of the North Atlantic Treaty Organization, my Doppler homodyne laser scanning system is conceptually described. A CO.sub.2 laser produces a beam that is then conically scanned on the surface by means of an aperture shared germanium wedged prism. The laser beam that is scattered from the surface is received by the same scanner and is mixed with an offset laser beam. Through a subsequent process of FM discrimination and phase sensitive detection the various Cartesian coordinate vectors may be resolved.